1. Field of the Invention
The present invention relates to a technique for subjecting a target substrate to a plasma process, and specifically to a plasma processing technique for processing a substrate, using radicals and ions derived from plasma. Particularly, the present invention relates to a plasma processing technique utilized in a semiconductor process for manufacturing semiconductor devices. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices and FPDs, plasma is often used for processes, such as etching, deposition, oxidation, and sputtering, so that process gases can react well at a relatively low temperature. Parallel-plate plasma processing apparatuses of the capacitive coupling type are in the mainstream of plasma processing apparatuses of the single substrate type.
In general, a parallel-plate plasma processing apparatus of the capacitive coupling type includes a process container or reaction chamber configured to reduce the pressure therein, and an upper electrode and a lower electrode disposed therein in parallel with each other. The lower electrode is grounded and configured to support a target substrate (semiconductor wafer, glass substrate, or the like) thereon. The upper electrode and/or lower electrode are supplied with RF voltage through a matching unit. At the same time, a process gas is delivered from a showerhead provided on the upper electrode side. Electrons are accelerated by an electric field formed between the upper electrode and lower electrode and collide with the process gas, thereby ionizing the gas and generating plasma. Neutral radicals and ions derived from the plasma are used to perform a predetermined micro-fabrication on the surface of the substrate. In the process described above, the two electrodes function to form a capacitor.
The majority of ions in the plasma are positive ions, and the number of positive ions is almost the same as that of electrons. The density of the ions or electrons is far smaller than the density of neutral particles or radicals. In general, plasma etching is arranged to cause radicals and ions to act on the substrate surface at the same time. Radicals perform isotropic etching on the substrate surface by means of chemical reactions. Ions are accelerated by an electric field and vertically incident on the substrate surface, and perform vertical (anisotropic) etching on the substrate surface by means of physical actions.
Conventional plasma processing apparatuses are arranged to cause radicals and ions generated in plasma to act on the substrate surface with the same density distribution. In other words, where the radical density is higher at the substrate central portion than at the substrate peripheral portion, the ion density (i.e., electron density or plasma density) is also higher at the substrate central portion than at the substrate peripheral portion. Particularly, in parallel-plate plasma processing apparatuses described above, if the frequency of the RF applied to the upper electrode is set higher, when the RF is supplied from an RF power supply through a feed rod to the electrode backside, it is transmitted through the electrode surface by means of the skin effect and is concentrated at the central portion of the electrode bottom surface (plasma contact surface). As a consequence, the electric field intensity at the central portion of the electrode bottom surface becomes higher than the electric field intensity at the peripheral portion, so both the radical density and ion density (electron density) become higher at the electrode central portion than at the electrode peripheral portion. However, if radicals and ions are always limited or restricted to such a relationship that they have the same distribution in acting on the substrate surface, it is difficult to perform a predetermined plasma process on the substrate, and it is particularly difficult to improve the uniformity in process state or process result.